Dehydrochlorination of hydrochlorofluorocarbons using pre-treated activated carbon catalysts

ABSTRACT

The present disclosure provides methods for pre-treating activated carbon before it is used in a dehydrochlorination process. The methods can comprise mixing the activated carbon with an acid, an oxidizing agent in a liquid phase, or an oxidizing agent in a gas phase. Activated carbons undergoing one or more of these methods can exhibit improved stability during the dehydrochlorination process.

CROSS REFERENCE TO RELATED APPLICATION

This application is related to, and claims priority to, U.S. ApplicationSer. No. 60/963,913, filed Aug. 8, 2007 which is incorporated herein byreference.

BACKGROUND

1. Field of the Disclosure

The present disclosure relates to a method for using activated carbonsin dehydrochlorination processes. More specifically, the presentdisclosure relates to a method for preparing hydroflouoroalkenes fromhydrochlorofluoroalkanes using pretreated activated carbon.

2. Description of the Related Art

Activated carbons can be used as a catalyst for the dehydrochlorination,or conversion of hydrochlorofluorocarbons (HCFCs) into fluorinatedalkenes that have lower global-warming potentials (GWP). Thesefluorinated alkenes can be used in a wide variety of applications,including as refrigerants, propellants, cleaning agents, and as monomersof macromolecule compounds.

The activated carbon tends to become deactivated quickly, however, whichresults in a drastically reduced rate of conversion of the HCFCs. Thus,there is a need for a method or process to improve the stability ofactivated carbon during the dehydrochlorination process.

SUMMARY OF THE INVENTION

Applicants have found demineralizing and/or oxidizing an activatedcarbon catalyst unexpectedly stabilizes the catalysts during certaindehydrochlorination reactions, for example, dehydrochlorinating1,1,1,2-tetrafluoro-2-chloropropane (HCFC 244bb) to form2,3,3,3-tetrafluoropropene (HFC-1234yf).

Accordingly, in certain aspects of the invention provided is a methodfor producing a fluorinated alkene comprising dehydrochlorinating ahydrofluorochloroalkane in the presence of a stabilized catalyst,wherein said stabilized catalyst is selected from the group consistingof demineralized activated carbon, oxidized activated carbon, or acombination thereof.

In another aspect of the invention, provided is a method forpre-treating an activated carbon catalyst comprising demineralizing saidactivated carbon catalyst and oxidizing said activated carbon catalyst.

In yet another aspect of the invention, provided is an activated carboncatalyst prepared according to such a pre-treatment process.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-5 show experimental data concerning several embodiments of themethod of the present disclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

The present disclosure has advantageously discovered a novel method forimproving the stability of activated carbon (AC) during thedehydrochlorination of HCFCs having at least one hydrogen and at leastone chlorine on adjacent carbons. The AC can be pre-treated before beingutilized in the dehydrochlorination process according to the methodsdiscussed in greater detail below. As is shown in the provided data,this pre-treatment provides a substantial improvement in the stabilityand performance of the AC.

In a first embodiment, the AC is pre-treated with an acid at roomtemperature or higher. Preferred acids for this process includehydrochloric acid (HCl), hydrofluoric acid (HF), or a combination of thetwo. The pre-treatment with the acid comprises the following steps: 1)the AC is mixed with an aqueous solution of the acid, 2) the suspensionis stirred for at least a first period of time at room temperature orhigher and then filtered to separate the acid from the AC, 3) the AC iswashed with distilled water until substantially free of ions from theacid, and 4) the AC is dried for at least a second period of time at afirst temperature. The AC sample can be dried in air at a temperature ofabout 50° C. to about 120° C., or higher. The AC can also be dried inair at a temperature of about 100° C. to about 110° C. The first periodof time can be from about 0.5 hours to about 24 hours, or longer. Thesecond period of time can also be from about 0.5 hour to about 24 hours,or longer.

In a second embodiment, the pre-treatment of the AC can be carried outusing an oxidizing agent in a liquid phase. In this embodiment, thepre-treatment comprises the following steps: 1) the AC is mixed with anaqueous solution of the oxidizing agent; 2) the suspension is stirredfor at least a third period of time at room temperature or higher andthen filtered to separate the AC from the oxidizing agent, 3) the AC isdried for at least a fourth period of time at a second temperature, andthen 4) heat-treated in an inert gas, such as nitrogen, for at least afifth period of time and at a third temperature. The third and fourthperiods of time can also be from about 0.5 hour to about 24 hours, orlonger. In step 3), the AC can be dried at about 50° C. to about 120°C., or higher. In step 4), the fifth period of time can be about fromabout 0.5 hour to about 4 hours, or longer. The third temperature can befrom about 250° C. to 750° C. or higher. The third period of time canalso be 1 hour, and the third temperature can also be about 400° C. Forthe liquid phase, non-limiting examples of the oxidizing agent includenitric acid (HNO₃) and hydrogen peroxide (H₂O₂) aqueous solutions, orcombinations of the two.

In a third embodiment, the pre-treatment of the AC can be carried outusing an oxidizing agent in a gas phase. In this embodiment, thepre-treatment comprises the following steps: 1) the AC is loaded into areactor, 2) pure or diluted gaseous oxidizing agent is flowed throughthe reactor, and 3) at least a portion of the AC is oxidized for a sixthperiod of time at a fourth temperature. In step 2, the oxidizing agentcan be diluted with an inert gas, such as nitrogen. The concentration ofthe oxidizing agent in this diluted mixture can be from about 1% toabout 10%. In step 3), the sixth period of time can be from about 5seconds to about 12 hours or longer, or alternatively, about 2 hours.The fourth temperature can be from about 250° C. to about 750° C., orhigher. The fourth temperature can also be about 450° C. Generallyspeaking, a longer period of time is needed for a lower temperature.During this oxidation step, oxygen-containing groups are formed on thesurface of the AC, and a small fraction of the AC may be burned off atelevated temperatures due to deep oxidation. For the gas phase,non-limiting examples of the oxidizing agent include diatomic oxygen(O₂) and carbon dioxide (CO₂), or combinations of the two.

The AC can be pre-treated with any one of the above describedpre-treatments singly, or can be treated with any combination of thethree. For example, the AC can be pre-treated with HCl, followed by asecond pre-treatment with HNO₃. In addition, although theabove-described methods concern AC that is pre-treated before being usedin a dehydrochlorination process, the present disclosure alsocontemplates treating spent or deactivated AC with these methods, torejuvenate the AC. The deactivated AC can undergo the treatment methodsdescribed above, and then be used in a dehydrochlorination process oncethey have been rejuvenated.

There are a number of HCFCs that can be used in the dehydrochlorinationprocess of the present disclosure. Table 1 below shows a list ofpossible HCFCs and the resulting fluorinated alkenes that are producedby the dehydrochlorination process.

TABLE 1 HCFC Fluorinated alkenes CF₃CFClCH₃ (244bb) CF₃CF═CH₂ (1234yf)CF₃CHFCH₂Cl (244eb) CF₃CF═CH₂ (1234yf) CF₃CH₂CHFCl (244fa) CF₃CH═CHF(trans/cis-1234ze) CF₃CHClCH₂F (244db) CF₃CH═CHF (trans/cis-1234ze)CF₃CFClCH₂F (235bb) CF₃CF═CHF (Z/E-1225ye) CF₃CHFCHFCl (235ea) CF₃CF═CHF(Z/E-1225ye) CF₃CH₂CF₂Cl (235fa) CF₃CH═CF₂ (1225zc) CF₃CHClCHF₂ (235da)CF₃CH═CF₂ (1225zc) CF₃CFClCHF₂ (226ba) CF₃CF═CF₂ (1216) CF₃CHFCF₂Cl(226ea) CF₃CF═CF₂ (1216)

In any of the above-described embodiments, the HCFC that undergoesdehydrochlorination can be 1,1,1,2-tetrafluoro-2-chloropropane, alsoknown as 244bb, and the resultant fluorinated alkene is2,3,3,3-tetrafluoropropene, also known as 1234yf. The followingexperimental data demonstrates that pre-treatment of AC, before beingused in the dehydrochlorination process, can improve the ability of theACs to catalyze the conversion of HCFCs into fluorinated alkenes, overthat which is achieved with untreated AC. Methods of dehydrochlorinationare described in co-pending U.S. patent application Ser. No. 11/619,592,filed on Jan. 3, 2007, (hereinafter “the '592 application”) which isincorporated herein by reference. The AC is available from a number ofsources, including the Alfa Aesar Corporation.

EXAMPLE 1 244bb Dehydrochlorination Over Untreated and HCl-Treated ACs

In Example 1, untreated and HCl-treated activated carbons (ACs) wereused as dehydrochlorination catalysts. 20 cc of catalyst granules wasused. A mixture of 92.7% of 244bb/6.5% of 1233xf was passed through abed of each of the AC catalysts at a rate of 6 g/h. 1233xf is anintermediate product formed during the fluorination of CCl₂=CClCH₂Cl,and is used as raw material for producing 244bb, as described in the'592 application. For this reason, streams of 244bb often comprise someamount of 1233xf. The temperatures at the bottom of the catalyst bed andat the top of catalyst bed were recorded and reported. As shown in FIG.1, which shows data at 350-385° C., the stability of AC was slightlyimproved after treatment with HCl.

EXAMPLE 2 244bb Dehydrochlorination Over HCl- and HCl & HNO₃-TreatedActivated Carbons

In Example 2, AC pretreated with HCl, and AC pre-treated with both HCl &HNO₃, according to the methods described above, were used asdehydrochlorination catalysts. 20 cc of catalyst granules was used. Amixture of 92.7% of 244bb/6.5% of 1233xf was passed through a bed ofeach of the AC catalysts at a rate of 6 g/h. The temperatures at thebottom of the catalyst bed and at the top of catalyst bed were recordedand reported. As shown in FIG. 2, at 350-385° C., compared to the ACpre-treated with only HCl, the HCl & HNO₃ pre-treated AC showed muchhigher stability. Over the latter the conversion of 244bb was stillabove 80% after 10 hours on stream, while over the former it was alreadybelow 55% after 10 hours on stream. This result suggests oxidationtreatment in liquid phase with HNO₃, particularly when used inconjunction with the pre-treatment of HCl, can greatly improve thestability of AC.

EXAMPLE 3 244bb Dehydrochlorination Over HCl- and HCl & H₂O₂-TreatedActivated Carbons

In Example 3, AC pre-treated with HCl, and AC pre-treated with both HCl& H₂O₂, according to the methods described above, were used asdehydrochlorination catalysts. 20 cc of catalyst granules was used. Amixture of 92.7% of 244bb/6.5% of 1233xf was passed through a bed ofeach of the AC catalysts at a rate of 6 g/h. The temperatures at thebottom of the catalyst bed and at the top of catalyst bed were recordedand reported. As shown in FIG. 3, at 350-385° C., compared to the ACpre-treated with HCl only, the AC pre-treated with both HCl & H₂O₂exhibited higher stability. The latter exhibited a rate of conversion of244bb of about 70% after 10 hours on stream, while the former exhibiteda rate of below about 55% after 10 hours on stream. This result suggeststhat pre-treatment of the AC in liquid phase with H₂O₂, afterpre-treatment with HCl, can improve the stability of the AC over thatwhich is only pre-treated with HCl.

EXAMPLE 4 244bb Dehydrochlorination Over HCl- and HCl & 5% O₂/N₂-TreatedActivated Carbons

In Example 4, AC pre-treated with only HCl and AC pre-treated with HCl &a mixture of 5% O₂/95% N₂ were used as dehydrochlorination catalysts. 20cc of catalyst granules was used. A mixture of 92.7% of 244bb/6.5% of1233xf was passed through a bed of each of the AC catalysts at a rate of6 g/h. The temperatures at the bottom of the catalyst bed and at the topof catalyst bed were recorded and reported. As shown in FIG. 4, at350-385° C., the AC treated with HCl and the mixtures of 5% of O₂/95% ofN₂ was able to maintain its activity at the level of about 70% foralmost 7 hours (from the 3^(rd) to the 10^(th) hours on stream), atleast. The performance of the HCl-treated AC, in contrast, decreasedover time. This indicates that oxidation pre-treatment of the AC with agaseous O₂ mixture, particularly in conjunction with pre-treatment withHCl, can greatly improve the stability of ACs long term.

EXAMPLE 5 244bb Dehydrochlorination Over Pristine and HNO₃-TreatedActivated Carbons

In Example 5, untreated AC, and AC pre-treated with HNO₃ according tothe method described above, were used as dehydrochlorination catalysts.20 cc of catalyst granules was used in a typical run. A mixture of 97.2%of 244bb/2.0% of 1233xf was passed through catalyst bed at a rate of 6g/h. The temperatures at the bottom of the catalyst bed and at the topof catalyst bed were recorded and reported. As shown in FIG. 5, at350-385° C., the AC pre-treated with HNO₃ was able to maintain itsactivity at the level of above 75% from the 4th to the 8^(th) hour. Theperformance of the untreated AC, by contrast, steadily decreased overtime. This indicates that the stability of the AC can be significantlyimproved by pre-treatment with HNO₃, even without pre-treatment withHCl.

The present invention having been thus described with particularreference to the preferred forms thereof, it will be obvious thatvarious changes and modifications may be made therein without departingfrom the spirit and scope of the present invention as defined in theappended claims.

1. A method for producing a fluorinated alkene comprising:dehydrochlorinating a hydrofluorochloroalkane in the presence of astabilized catalyst, wherein said stabilized catalyst is selected fromthe group consisting of demineralized activated carbon, oxidizedactivated carbon, or a combination thereof.
 2. The method of claim 1wherein said hydrochlorofluoroalkane is selected from the groupconsisting of 1,1,1,2-tetrafluoro-2-chloropropane,1,1,1,2-tetrafluoro-3-chloropropane,1,1,1,3-tetrafluoro-3-chloropropane,1,1,1,3-tetrafluoro-2-chloropropane,1,1,1,2,3-pentafluoro-2-chloropropane,1,1,1,2,3-pentafluoro-3-chloropropane,1,1,1,3,3-pentafluoro-3-chloropropane,1,1,1,3,3-pentafluoro-2-chloropropane,1,1,1,2,3,3-hexafluoropropane-2-chloropropane, and1,1,1,2,3,3-hexafluoro-3-chloropropane.
 3. The method of claim 1 whereinsaid hydrochlorofluoroalkane is 1,1,1,2-tetrafluoro-2-chloropropane. 4.The method of claim 1 wherein said fluorinated alkene is selected fromthe group consisting of: 2,3,3,3-tetrafluoropropene,1,3,3,3-tetrafluoropropene, 1,2,3,3,3-pentafluoropropene,1,1,3,3,3-pentafluoropropene, and 1,1,2,3,3,3-hexafluoropropene.
 5. Themethod of claim 3 wherein said fluorinated alkene is2,3,3,3-tetrafluoropropene.
 6. The method of claim 1 wherein saidstabilized catalyst is activated carbon that has been demineralized inthe presence of at least one acid selected from the group consisting ofhydrochloric acid and hydrofluoric acid.
 7. The method of claim 6wherein said acid is hydrochloric acid.
 8. The method of claim 1 whereinsaid stabilized catalyst is activated carbon that has been oxidized inthe presence of at least one liquid phase oxidizing agent selected fromthe group consisting of nitric acid and hydrogen peroxide.
 9. The methodof claim 1 wherein said stabilized catalyst is activated carbon that hasbeen oxidized in the presence of at least one vapor phase oxizidingagent selected from the group consisting of O₂ and CO₂.
 10. The methodof claim 6 wherein said activated carbon further has been oxidized inthe presence of nitric acid.
 11. The method of claim 6 wherein saidactivated carbon further has been oxidized in the presence of hydrogenperoxide.
 12. The method of claim 6 wherein said activated carbonfurther has been oxidized in the presence of O₂.
 13. The method of claim1 wherein said stabilized catalyst is activated carbon that has beensubjected to a demineralization process comprising the following:forming a suspension comprising said activated carbon and at least oneacid selected from the group consisting of hydrochloric acid andhydrofluoric acid; stirring said suspension at a temperature of at leastabout 22° C. for a first period of time from about 0.5 to about 24hours; filtering said suspension, to separate said activated carboncatalyst from said acid; washing said catalyst to remove substantiallyall free ions from said activated carbon catalyst; and drying saidcatalyst at a temperature of at least about 50° C. for a second periodof time from about 0.5 to about 24 hours.
 14. The method of claim 1wherein said stabilized catalyst is activated carbon that has beensubjected to a liquid phase oxidation process comprising the following:forming a suspension comprising said activated carbon and at least oneoxidizing agent selected from the group consisting of nitric acid andhydrogen peroxide; stirring said suspension at a temperature of at leastabout 22° C. for a first period of time from about 0.5 to about 24hours; filtering said suspension, subsequent to said stirring, toseparate said activated carbon from said oxidizing agent; drying saidactivated carbon, subsequent to said filtering, at a temperature of atleast about 50° C. for a second period of time from about 0.5 to about24 hours; and heat-treating said catalyst in an inert gas for a thirdperiod of time from about 0.5 to about 4 hours at a temperature of about250° C. to about 750° C.
 15. The method of claim 1 wherein saidstabilized catalyst is activated carbon that has been subjected to avapor phase oxidation process comprising the following: contacting saidactivated carbon with said gaseous oxidizing agent at a temperature ofabout 250 to about 750° C. for a period of time from about 5 seconds toabout 12 hours.
 16. A method for pre-treating an activated carboncatalyst comprising: demineralizing said activated carbon catalyst, andoxidizing said activated carbon catalyst.
 17. The method of claim 16wherein said demineralizing involves contacting said activated carboncatalyst with at least one acid selected from the group consisting ofhydrochloric acid and hydrofluoric acid.
 18. The method of claim 16wherein said oxidizing involves contacting said activated carboncatalyst with at least one oxidizing agent selected from the groupconsisting of nitric acid, hydrogen peroxide, O₂, and CO₂.
 19. Anactivated carbon catalyst prepared according to the process of claim 16.